1. Field of the Invention:
The present invention relates to an apparatus for externally injecting ions into a cyclotron.
2. Description of the Prior Art:
Generally, in a cyclotron, an ion source is disposed at the center of magnetic poles. Therefore, due to the fact that a degree of vacuum is deteriorated by discharge of gas from the ion source and an orbit of an ion beam is varied as a result of change of operation parameters of the ion source, acceleration and take-out of a large-intensity ion beam are difficult. In order to obviate this difficulty, heretofore, various methods for externally injecting ions into a cyclotron have been proposed, as described in the following:
(1) Perpendicular injection method
This method is such that a hole is drilled at the center of the magnetic pole, then an ion beam is injected in perpendicular to an acceleration orbit plane along which the ion beam is to be accelerated (since this plane is present on a midplane of a D.C. magnetic field, it is also called "magnetic midplane"), and when the ion beam has approached the acceleration orbit plane, the direction of the ion beam is changed by means of an electric field to be driven into the acceleration orbit plane. For the purpose of changing the direction of the ion beam, various implements such as an extracting electrode and a fine adjustment therefor are disposed at the central portion of the magnetic poles.
Except for the above-mentioned perpendicular injection method, the methods described hereunder are methods of injecting an ion beam into a cyclotron within an acceleration orbit plane (horizontal plane).
(2) Method of straightly injecting ions by offsetting the force exerted upon the ion by a magnetic field with an electric field
This method was tried at the Saclay Research Laboratory in France, and about in 1968 they succeeded in acceleration of a polarized proton beam by means of a cyclotron for the first time in the world. In this method, it is necessary that a high voltage is used in a narrow magnetic pole gap and the electric field intensity is varied according to variation of the magnetic field along the orbit.
(3) Gladyshev method
This method is called so because about in 1967, Gladyshev of USSR announced his success. Thereafter, success is also reported by making use of the cyclotron at Delft in Holland. According to this method, an ion beam is made to perform circular motion many times until it reaches the central portion of magnetic poles as it travels along the boundary between a hill and a valley of a magnetic field in an AVF cyclotron.
(4) Neutralized particle injection method
This method is such that initially an ion beam is accelerated under the condition of ions, after the traveling direction of ions has been determined by means of an electromagnetic field the ions are made to pass through a gaseous medium to be neutralized, then they are made to travel straightly by avoiding the influence of a magnetic field and are made to pass through a thin film stripper disposed at the central portion of the magnetic poles to be reionized, and they are accelerated. This method was tried in a cyclotron in Yugoslavia.
(5) Method of modifying a structure of a cyclotron per se so as to be adapted for horizontal injection
A separated-sector type cyclotron has the modified structure. The portion of a valley in the conventional AVF cyclotron is removed and separated into individual magnets, and thereby the magnetic field strength at the valley is made to be zero. An externally injected ion beam would travel straightly through the valley portion, then it changes a traveling direction at the central portion of the magnetic field, and it is accelerated. Through this method, acceleration of a high-energy beam having a large intensity, which was impossible with the AVF cyclotron, becomes possible.
However, the above-mentioned methods in the prior art have the disadvantages as will be described in the following:
At first, in the method (1) above, since the hole at the center of the magnetic pole through which an ion beam passes has to be small in diameter and hence evacuation through this hole is difficult, it is impossible to attain a high vacuum. Hence, there exists the inconvenience that electric charge of a heavy ion or a negative hydrogen ion would change. In addition, it is difficult to design an electrode group for changing the direction of an ion beam because the environmental magnetic field is varied due to the influence of the hole in the magnetic pole, and unless the gap space between the magnetic poles is large, it is impossible to dispose the electrode group therein. Therefore, this method (1) above can be employed only in a large-sized cyclotron.
In the method (2) above, a high voltage is used in a narrow magnetic pole gap, and since the direction of the ion beam would change unless the electric field intensity is finely adjusted, this method is disadvantageous in that loss of an ion beam would become large due to generation of electric discharge or presence of small field error in the magnet. Accordingly, at present, this method (2) is not employed.
In the method (3) above, as the ion beam performs circular motion many times until it reaches the central portion of magnetic poles, it is inevitable that the ion beam would diverge before it reaches the central portion of magnetic poles and hence the proportion of attaining incidence to the acceleration orbit would be decreased. In addition, this method is disadvantageous in that even with a slight change of the magnetic field strength, the orbit of the ion beam in the proximity of the central portion of magnetic poles would change largely, and it takes much time for readjusting the magnetic field strength. Consequently, this method is also not employed at present.
In the method (4) above, there results a very small product of a probability in the electric charge transformation process from ions to neutralized particles and another probability in the electric charge transformation process from the neutral particles to ions. In addition, particles would scatter at the respective transforming sections and an ion beam would diverge, and hence a strength of the beam which can be accelerated would become extremely small. Accordingly, this method is also not used at present.
In the method (5) above, the number of magnets is so many that the weight of the cyclotron would become large as compared to an AVF cyclotron of the same energy. In addition, high precision is required for manufacture and installation, and hence a cost would become high. Accordingly, this is a method suitable for a large-sized high-energy cyclotron.
As described above, an external ion injection method is not used for a small-sized AVF cyclotron at present. Although the perpendicular injection method is employed in a large-sized AVF cyclotron, there exist various problems as mentioned above, and the method has not been successfully employed in injection of a large-intensity beam to a standard type AVF cyclotron.